Man...I could have written the exact same thing...word for word. My alk is higher than I would like (10) for exactly this reason. I've learned some very interesting stuff about my Kalk reactor as I've tried to use it to prevent overnight pH lows. (when you are hitting nightly lows of pH 7.6...you just can't ignore pH at that point)

The way I see it is if you're pH is low at a natural alk level you've got other problems to address that aren't really solved by simply adding more carbonate to increase alkalinity. More than likely the amount of CO2 in the water is too high and instead of covering this issue with something else you should be looking at the source of the problem. 99% of the time it comes down to the calcium reactor for obvious reason, it runs on CO2. The problem is very few understand the science behind these reactors and simply set an internal or effluent pH and figure that's good. However there is much more to consider including media type, reactor design, effluent flow rate, circulation flow rate, and CO2 flow rate.

For example, you find your tank pH is lower than normal say 7.8. First off you know that tanks using a Ca reactor will usually have a little lower pH than tanks using other methods, but I don't think lower than 8.0 mid-day is normal. So what's the problem? Too much CO2? Most likely but what's the solution? Most would simply bump the alkalinity up by increasing the output of the reactor by increasing the CO2 input which will actually add even more CO2 to the tank. What you should do is adjust your reactor, you've obviously got too much dissolved CO2 coming out of the reactor, meaning either too much CO2 is being added, flow rates are too high or a combination of both. The reactor may also be too small for the demand of the system or be of poor quality and design.

Another alternative to adding more CO2 would be to add kalkwasser instead which actually reacts with the CO2 in the tank promoting a higher and stable pH.

I don't think anything over 10 dkh is good for SPS, mainly because I've never seen a successful SPS tank with an alkalinity that high. Even if you go through RC TOTM I don't think you'll see anything with much higher than 9 dkh.

Once you have concluded that your low pH is caused by too much CO2...solving that is difficult. My tank is just too sealed up and doesn't breathe well. The herbie overflows eliminate a lot of natural aeration that would normally help. It is a brand new house that is tight and even in the summer the air conditioning keeps the house cool and closed up. I need to feed my skimmer outdoor air somehow.

In my case, the Ca reactor has been shut off for quite a while and Kalk alone has driven up my Alk..thereby solving my low pH problem for the time being.

A number of my coral lay down more tissue at peak pH above 8.2 than they did when I couldn't get the pH above 8. I don't think there is much doubt that higher pH is better than lower (to a point of course).

I would be happier at an Alk around 8-9...but I'll push that higher in order to keep the pH above 8.

I'm actually fine right now, my pH is in the 8.0-8.2 range. Two days ago I measured my alkalinity (Elos test kit) and I was sitting at 10dkh. Like I was saying, in the winter my house is too closed up and I have a lot of CO2 build up which drives down my tanks pH. In the summer I open up the downstairs windows and everything is fine. I dont think many of those guys on the Reef Central TOTM live up in the frozen north like I do. Many of their tanks are running chillers while my heater is still kicking on each day.

I run an old down flow PM 622 Calcium Reactor with course ARM media and a DIY 6" diameter 1' tall effluent chamber with fine ARM media. pH in the reactor is between 6.5-6.6 and controlled by an AquaController. There is enough flow through to maintain my tanks calcium and alkalinity at constant rates of ~420ppm and 11dkh respectively. I also run all of my top off (about 2 gal a day) through a knock off of a PM Kalk Reactor.

Do you see anything that I could do differently with regards to my reactor?